In the course of maintenance and repair of telephone line circuits it is common practice for the craftsperson to couple a test set to the line, monitor the line for the presence of audible traffic and, in the absence of a detectable audio signal, place the test set in a low impedance "TALK" mode in order to use, test, or otherwise carry out communication capability tasks on the accessed circuit. If, during this procedure, the telephone line is carrying information signals (e.g. digital data traffic) that lie in a frequency range above the audio spectrum, the craftsperson will be unable to detect the transmission. As a consequence, when the craftsperson seizes the line, (by placing a low impedance across its tip and ring leads) there is the likelihood of contamination of the out-of-band digital data. Moreover, with conventional test sets it is possible to be in the low impedance mode at the time of connection to the circuit thereby interfering with all traffic which might be present.